ALD is a method of depositing a thin film on a substrate in a very controlled manner. The deposition process is controlled using two or more chemicals in vapor form (i.e., “process gasses”) and reacting them sequentially and in a self-limiting manner on the surface of the substrate such as a silicon wafer. The sequential process is repeated to build up the thin film layer by layer, wherein the layers are atomic scale.
ALD is used to form a wide variety of films, such as binary, ternary and quaternary oxides for advanced gate and capacitor dielectrics, as well as metal-based compounds for interconnect barriers and capacitor electrodes.
The typical ALD process introduces a first process gas into the single process chamber. The substrate sits in this environment for a short period of time to expose the surface to the first process gas. Once the surface of the substrate is saturated with the first process gas, the first process gas is pumped out of the chamber. Then an inert or purge gas is flowed through the chamber. Then a second process gas is introduced into the chamber. The second process gas reacts with the surface of the substrate that was saturated by the first process gas. The purpose of the inert or purge gas flowing through the chamber before the second process gas is flowed into the chamber is to ensure that all of the original unreacted first process gas is removed. The second process gas reacts with the surface of the substrate that was saturated with the first process gas.
Once this second reaction process is completed, the second process gas is removed and the chamber is again purged with an inert or purge gas. Then, the first process gas is introduced into the chamber and the entire reaction sequence is repeated until an ALD film of a desired thickness is obtained.
In one type of ALD process, the ALD film covers the entire surface of the substrate. However, when forming a semiconductor device, it is typically preferred to form the ALD film in select areas of the substrate in the process of defining a 3D semiconductor device structure. While this can be done using physical masking methods, such as photoresist-type masking, such masking methods are process-intensive and time consuming.